Speed stropping

[matt321]

Been reading the thread about belt sander sharpening and doing some mind-movie experiments. Someone commented that power-machined surfaces are smoother than hand finishing at the same grit, and I was wondering why that might be.

Invision a coarse abrasive belt moving at very high speed. The apparent effect would be that of a less coarse belt. If the grains are stepping forward quickly in succession, then the surface can't feel the full depth of the grit.

So I decided to try some high speed hand-stropping on leather loaded with CrO. I held the angle of attack as best I could and took care to avoid raising the spine at the end of the stroke. Each stroke was a fast skimming attack on the strop. I made the strokes as light as possibe.

The results showed the edge went from 4 inch push-cut to 4.5 inch on printer paper. Is this real or placebo?

 

[Gena]

Sorry for ignorance, what is 4 inch push-cut? Pushing straight down to make a 4" long cut?

 

[Cliff Stamp]

You are seeing a 10% difference, I would argue that if you combine the expected deviations of both hand sharpening with the inherent deviations in the steel itself, then one shot comparisons will have larger variations. In short, that isn't strong evidence of a correlation.

I would look at an average of at least 4-5 sharpenings in both ways and see if they were significantly different. A rough comparison is to see if the averages overlap when you consider the standard deviations. Now push cutting paper is somewhat subjective unless you establish very tight controls on method so the ideal way is to have someone who doesn't know which way the blades were sharpening do the cutting.

-Cliff

 

[sodak]

Sorry for ignorance, what is 4 inch push-cut? Pushing straight down to make a 4" long cut?

If you are right handed, hold the paper (newspaper or phone book page ripped out) vertically in your left hand. Using just a push cut with the knife, see how far away from your left hand's pinch on the paper you can cut. 4 inch is about the best I've ever done, usually 2 to 3 is my average on newsprint.

 

[matt321]

There is a concensus that the effect of a belt sander cannot be duplicated by using the same belt in a stationary way and just working the edge longer to make up for the lost speed. So speed must effect the properties of the belt (i.e. apparent grit and apparent sharpness of the grit).

Still, most manual stropping discussions say speed is not important or that speed should be avoided. So there seems to be a conflict. Don't barbers strop razors with a very rapid stroke?

My little experiment tested two knives. Both had been sharpened through fine grits (ultra-fine sharpmaker) and both had been maticulously stropped on a rigid, flat surface going very slow with light pressure and attention to correct angle. They were as sharp as I could get them. Now, adding speed to the stropping stroke seems to have added a little more sharpness (as best I can tell without a more scientific testing regimen).

The speed stropping was like striking a match on a matchbox. Either start by setting the edge lightly on the strop, adjust the angle, then accelerate rapidly across the strop. Alteratively, make a sweeping arc which contacts the strop at speed.

 

[Cliff Stamp]

There is a concensus that the effect of a belt sander cannot be duplicated by using the same belt in a stationary way and just working the edge longer to make up for the lost speed.

There was a concensus that the earth was flat. Religion is ran by concensus, science is ran by FACTS. I sharpened a knife on a belt sander and then by hand on the same belt several times. I then did cutting trials on 3/8" rope and MEASURED the initial sharpness and the sharpness during the cutting. There was no difference. I repeated this for multiple knives. I also did independent verification, had someone else do runs, where they didn't know what edge was from what.

-Cliff

 

[ferguson]

I do all of my sharpening on a Coote 2x72" belt grinder. Cause I'm lazy. To strop, I turned a worn out belt inside out and rubbed on green chrome compound. I then strop lightly on the slack belt. It makes a very finely polished edge. There is still a microburr left on the edge, that get's taken of with a piece of leather glued to a board. Never did any testing. And I'm not sure if this is relevant to the thread. :foot:

In grinding blades, I have noticed that 220 on the belt gives a much finer finish that 220 paper by hand. Don't know if the speed is the factor, or if it's due to the fact that you sanding so much more, and lighter, with the machine.

Steve

 

[Ripantuck]

The way I understand it is that this effect is caused by putting a light amount of pressure on the belt (or wheel) and then leaving it there for a second or so. That is; don't keep pushing but keep the blade in the same spot.

This causes the grains to knock off any high spots without going any deeper. My understanding is that's the reason automated grinders are programmed to put the blade to be ground in an exact spot- then either the blade or the grinder goes left and right when the belt or wheel grinds into the blade, it stops grinding for a mere fraction of a second in any one spot, but long enought to knock off any high spots.

This is, at least, part of the reason for the advice in hand sharpening


to take lighter and lighter passes.

Sorry if I didn't explain this well. Are there any people here who work for a company that uses automated grinders, i.e. planer blade manufacturers, router bit manufacturers, precision engine parts, etc.?

Don